Current transformer



D. C. PRINCE CURRENT TRANSFORMER Filed July l2, 1954 Aug. 26, 1958 2,849,694: Patented Aug. 26, 1958 lice CURRENT TRANSFORMER David C. Prince, Schenectady, N. Y., assigner to General Electric Company, a corporation of New York Application July 12, 1954, Serial No. 442,470 2 Claims. (Cl. 336--'174) This invention relates to a high voltage current transformer and, more particularly,to the type of transformer which is provided with a loop-shaped primary winding disposed within an insulating housing.

To meet the severe insulation requirements present in such a transformer, it has been conventional in the past to form the loop-shaped primary structure of the transformer in the shape of an eye-bolt with the primary conductor coated by a thick unitary, insulating sheath. An example of this construction is illustrated in U. S. Patent No. 2,3G0,9l`-Camilli, assigned to the assignee of the present invention. Not only is the insulation coating process for such a primary quite costly because of the intricate `configuration of the primary conductor, but the resulting primary structure does not lend itself to being readily linked with the usual core and secondary winding of the transformer. With such a primary construction, the core, which is a closed annulus, cannot be preformed, but must be formed with the looped primary structure present at its center during the formation process. The presence of the primary conductor not only complicates the process of core formation but also complicates the process of linking the secondary winding to the core.

Accordingly, it is an object of the present invention to provide an improved current transformer which readilylends itself to being constructed and assembled at very low cost.

It is a further object of my invention to provide a current transformer constructed in a manner which readily permits its various components to be preformed and subsequently assembled with a minimum of difficulty.

Another object is to construct and arrange the components of the transformer in a manner which not only fulfills the necessary insulation requirements but also permits the components to be enclosed by a housing of a small and very compact form.

In accordance with one form of my invention, the current transformer assembly includes a pair of elongated bushing cores closely surrounded by a ceramic insulating shell. The bushing 'cores are mounted to extend throughout their lengths in closely-spaced, substantially-parallel relationshipl Each of the bushing cores comprises an elongated conductive stud extending in an essentially rectilinear path and an insulating sheath `disposed about the stud. The studs are connected in series circuit relationship by a conductive member detachably clamped across their ends. Mounted/about one of the sheaths is an annular magneticV core having a secondary winding inductively linked thereto. f

The invention will be best understood by referring to the drawing in which Fig. l is a cross-sectional View of one form of current transformer embodying my invention. Fig. 2 is a fragmentary View illustrating the adaptability of the current transformer of Fig. l to use with shielding structure.

Referring to Fig. l of the drawing, the transformer is provided with an enclosing housing 1t) comprising a grounded metallic tank 11 and a tubular ceramic shell 12 (preferably of porcelain) supported on the tank by means of a vcover 13. VThe cover 13 and the tank 11 are providedr with anges suitably clamped together in sealing relationshipby a series of bolts 14. Supported at the upper end of the porcelainshell 12 isfan enclosing dome-- 15 having an annular flange 16 providing a seat for supporting the dome onthe porcelainshell l2. The dome and the porcelain shell are maintained in assembled relationship on the cover 13 by tensioning means in the form of suitable elongated insulating bolts such as 17 acting through a clamping plate, or compression member, 18 which bears against the seating-flange 16 of the dome 15. The housingy 10 is filled with a suitable insulating uid to a level 19, as indicated near the top in Fig. l.

In accordance with my invention, the primary winding of the transformer comprises a pair of bushing cores 2t) and 21 which are interconnected at their lower ends` by a conductive strap 22. Each `of these bushing cores cornprises an elongated tubular stud about which there is disposed an insulatingsheath-24. -These studs are respectively ydesignated as`23 and 23a. Each of the sheaths 24 is preferably formed of'superimposed'layers of insulating paper tightly..wound aboutl the stud as a center. The winding process'is carried out prior to assembly of the bushing core within the` housing 10 and-is considerably facilitated by the factthat eachof the studs has an essentially rectilinear aXis. v-As a result, the winding processis a simple-operation-which-may be performed by machine simply vby turning the core at high speed whereby to wind the insulatingpaperthereabout Because of the simple rectilinear, configu-rationof the stud, I have obviated theneed forthegmanual winding operation which has generally been found necessary in .order to apply insulationfto the primary'of those transformers having an eyebolt-shaped primary..

For electrically.connectingtheconductive strap 22 to the lower ends of thestuds 23 and 23a, each of the studs is providedwith apair of clamping nuts'25 threaded thereon. These nuts are arranged to tightly clamp the strap Z2 therebetweeny so as to lprovide a good electrical connection between the studs.

To support the bushing cores within housing 10, the upper end of each Studis provided with a pair of clamping nuts 30 suitably threaded thereon. These nuts are arranged to tightlyclampvthe.supportingl plate 18 therebetween, thereby to rigidly `suspend the bushing cores from this plate 18. Suitable leads-for the primary lare providedby means of a pair of threaded terminal members 31 and 32 mounted at opposite sides of the dome 15. The terminal member 32 is insulated from the dome 15, as by an insulating gasket 33,

Although not-limited to such a construction, the transl former of my invention is readily adapted fork use as a current transformer having a multiple turn` primary winding. These multiple turns `may, readily .be provided by utilizing the open-ended tubular studs 23, 23a as a single turn which houses a second turn in ay generally coaxial relationship. For example, one turnof the primary winding is generally constitutedvby theseries-connected tubular stud 23a, strap 22, andv tubular stud 23, whereas the second turn of the primaryv is constituted by a exible conductor 34 extending, coaxially' and downwardly through tubular stud 23a, across the lower open ends of the studs, and then coaxiallyandupwardly through stud 23. At one of itsendsthe flexible conductor 34 is electrically connectedlin series circuit relationship to the stud 23 by a lug 35, whereas yat its other end the conductor is electrically joined to terminal member 32 by .suitable conductive connecting structure 36. Thel stud 23a is connected to the oppositeterminal member 31 by means of a lug 37. To prevent short- ,circuiting of the primary turns, the conductor 34 is provided with a suitable insulating covering. Since the Voltage between the primary turns is comparatively small, this insulating coveringmay be relatively thin.

If more than two'turns is required for the primary 0f la the current transformer, the flexible conductor could be wound once again through the tubular studs 23a, 23 before being connected to terminal member 32.

Alternatively, if only a single turn is required for the primary, the flexible conductor 34 can be omitted and the upper ends of the studs 23a and 23 directly connected to the respective terminal bolts 31 and 32.

The secondary structure of the transformer comprises an annular magnetic core 40 about which is wound a multiple-turn secondary winding 41 having suitable terminal leads 42. The turns of the secondary winding are insulated from each other and from the core 40 in a conventional manner. Since it is possible to assemble this secondary structure about the primary simply by sliding the secondary structure over the lower end of the bushing core 23, it is therefore possible to preform the magnetic core 40 and to link the secondary Winding 41 to the core prior to assemblying this structure in its final position about the primary. Since this preformation of the secondary may be accomplished without any interference from the primary structure about which 4it is to be later assembled, such preforrnation is a simple operation which may be carried out at high speed and low expense essentially by machine. Thus, after the secondary structure has been completed, it is slid over the lower end of the bushing core 23 and is positioned thereabout by suitable mounting structure (not shown). This assembly operation takes place before the lower ends of the bushing cores are interconnected by the strap 22 or the cable 34. Then, after the secondary structure is properly positioned, the detachable strap 22 is clamped in place across the lower ends of the cores and the flexible conductor 34 is drawn through the tubular studs.

As described in Application Serial No. 432,127 filed on May 25, 1954, now Patent No. 2,804,577, by A. W. Roth and assigned to the assignee of the present invention, it is often necessary to provide the primary structure with grounded shielding in order to insure correct energization of the secondary winding (41) in response to faults which might occur within the housing 10. The current transformer assembly of my invention readily lends itself to the incorporation of such shielding. For example, as shown in the Roth application and as illustrated in Fig. 2 of my drawing, a conductive sleeve 50 may be mounted concentrically about each of the cylindrical bushing cores, and these sleeves may be interconnected by suitably grounded conducting structure 51. Since each of the bushing cores extends `in an essentially rectilinear path, these conductive sleeves may take the form of simple, cylindrical metal tubes. Since the conductive structure 22, 34 bridging the studs 23, 23a is removable, the shielding may readily be incorporated into the current transformer simply by sliding the composite shielding structure over the lower ends of the bushing in a manner similar to that described for assemblying the secondary structure 40, 41.

Although, as described above, the primary structure of my transformer readily lends itself to use with shielding 50, 51, it is to be understood that this shielding forms no part of my present invention. Such shielding structure and its relationship to other parts of the electrical system are described and claimed in the aforesaid Roth application.

To provide a uniform distribution of high voltage stresses throughout each of the insulating sheaths 24, a series of generally -concentric electrostatic shields S3 are provided within each of the sheaths. These shields may conveniently be formed by providing the insulating paper from which the sheath is wound with spaced areas coated with a conductive composition. A high voltage bushing core which is provided with such electrostatic shields is generally known as a capacitance type bushing core. Because the low voltage parts such as cover 13 and secondary structure 40, 41 are in closest proximity to the bushing cores at the axially central regions of the '4, cores, it is in this region that each insulating sheath 24 has its greatest wall thickness.

Bushing cores constructed and arranged in accordance with the present invention permit the use of a very compact insulating housing. For example, since the bushing cores extend in straight line paths they may be arranged in closely-spaced, substantially-parallel relationship. The only limitation on the closeness of the spacing is that sutlicient space must be allowed for the radial thickness of the secondary structure 40, 41. With the cores in this closely-spaced, substantially-parallel relationship the diameter of both the tank and the porcelain shell may be minimized. Additionally, since the secondary structure is mounted within the tank instead of within the porcelain shell, this secondary structure offers no obstacle to reducing the diameter of the shell to the minimum required to enclose the bushing cores.

While l have shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

Jhat I claim as new and desire to secure by Letters Patent of the United States is:

1. In a current transformer assembly, a pair of elongated bushing cores7 each having a rectilinear configuration and comprising an elongated conductive stud and an insulating sheath disposed thereabout, means for mounting said cores to extend throughout their lengths in closely-spaced, substantially-parallel relationship, a singular tubular ceramic insulating shell closely surrounding said pair of bushing cores, said core-mounting means comprising an insulating plate secured to one end of said ceramic insulating shell and means rigidly clamping said studs to said plate adjacent one end of said studs to thereby suspend said studs from said plate, a conductive member detachably secured to the other end of said studs and electrically interconnecting said studs in series circuit relationship, an annular magnetic core mounted about one of said sheaths, and a secondary winding inductively linked with said magnetic core.

2. In a current transformer assembly, a pair of elongated bushing cores, each having a rectilinear conguration, means for mounting said cores to extend throughout their lengths in closely-spaced, substantially parallel relationship, a metal tank surrounding the lower portions of said bushing cores, a cover mounted on said tank, a tubular ceramic shell mounted upon said cover and closely surrounding the upper portion of said pair of bushing cores, each of said bushing cores comprising an elongated conductive stud and an insulating sheath dis posed thereabout said core mounting means comprising a compression member seated atop said ceramic shell and extending transversely thereacross, means for securing said studs to said compression member, and tension means extending between said cover and said compression member for applying to said compression member a force which acts to clamp said compression member in position and to place said ceramic shell in compression; a conductive member detachably interconnecting said studs, an annular magnetic core mounted within said metal tank and about of of said bushing cores, and a secondary winding inductively linked with said magnetic core.

References Cited in the lile of this patent UNITED STATES PATENTS 1,808,533 Eby June 2, 1931 2,568,003 Hartmann Sept. 18, 1951 FOREIGN PATENTS 308,406 Great Britain Mar. 28, 1929 340,269 Great Britain Dec. 24, 1930 571,119 Great Britain Aug. 8, 1945 

